A tactical team moves through the mall parking lot at Clackamas Town Center in Portland, Ore., Dec. 11. Witnesses say the scene went from cheery to chaotic in seconds when a gunman opened fire in the suburban Portland shopping mall Tuesday, killing two people and wounding another.

Citizens' coolheadedness and individual preparation for coping with gunfire in public settings may have curtailed the casualty count from Tuesday's shooting at a Portland, Ore., shopping mall, law officers suggested on the day after the tragedy.

Two people died and one was critically wounded before the shooter, 22-year-old Jacob Tyler Roberts of Portland, killed himself a few minutes after running into the food court at the Clackamas Town Center mall. Officials say Mr. Roberts, wearing camouflage and a white hockey mask, had methodically fired "multiple" rounds from an assault-style rifle at random shoppers.

"Many people have asked me why there were so few victims during this incident," said Sheriff Roberts. He listed the fact that Mr. Roberts's AR-15 semiautomatic rifle intermittently jammed and noted a well-practiced mall lockdown procedure. But he also credited "10,000 people in the mall who at one time kept a level head, got themselves out of the mall, helped others get out, secured themselves in stores.… It was really about a whole group of people coming together to make a difference."

Law officers said during a Wednesday press conference that they did not know whether any member of the public carrying a concealed weapon had counterattacked Roberts. But they said they are certain that Roberts died by his own hand after fleeing down a stairwell from the mall's upper level.

The death rate from mass shootings has ticked up slightly in recent years, even as deaths in single-victim incidents have decreased, according to a recent analysis of FBI crime data by the Huffington Post. The worst recent mass shooting came in July in Aurora, Colo., where a gunman killed 12 people and injured 58 during a midnight screening of a new "Batman" movie.

Gun-control advocates seized on the mall shooting as a possible result of the expiration in 2004 of a national ban on assault weapons.

"Santa Claus could have been shot in the mall," said Penny Okamoto, executive director of Ceasefire Oregon, in an interview with the Portland Tribune. "If you're sick of this, you should call your legislators to tell them to fix the laws so that assault weapons don't end up in the hands of felons."

Many versions of the AR-15 were banned under the assault weapons law, but it's not known if the gun used in the Clackamas mall shooting was one of them.

Police said Roberts had no criminal record and had stolen the AR-15 from "someone he knew."

Does the collected response by shoppers at the Clackamas Town Center indicate that Americans are becoming less daunted by senseless violence and, perhaps, better ready to react? Those who back broad gun rights under the Constitution's Second Amendment suggest a shift may be under way in people's readiness to respond.

In blocking Illinois's ban on concealed weapons, the last such law in the nation, Seventh US Circuit Court of AppealsJudge Richard Posner on Tuesday implied that self-defense readiness in public is not only protected by the US Constitution, but may be good social policy. An awareness "that many law-abiding citizens are walking the streets armed may make criminals timid," he wrote in his ruling.

"As far as a social shift, I think people are getting more intelligent and appropriate in their reactions to shooters," says Dave Kopel, research director at the Independence Institute, a libertarian-leaning think tank in Golden, Colo. "Police training has changed in significant ways since the Columbine [High School] shooting [in 1999], where they no longer wait for the SWAT team to arrive but go in immediately with … the army they have. There's also an awareness [among police and the public] that if you're trying to stop a gangster from robbing a liquor store, you may have a [heck] of a fight on your hands, but that these publicity-seeking guys with mental illness, they basically crumble at first opposition."

The upshot, says Mr. Kopel: "Lying down and cowering doesn't seem to work very well, so law enforcement has gotten smarter and civilians have gotten smarter."

In Clackamas County, Sheriff Roberts said local law-enforcement personnel had trained earlier this year for a shooting scenario at Clackamas Town Center, an exercise that involved both police and retailers. On Tuesday, arriving police, in keeping with evolving police tactics nationwide, formed small teams and quickly entered the mall to pursue the shooter. Police could not say Wednesday whether any officers saw the shooter before he killed himself.

Dennis Curtis, the mall's general manager, noted that police officers told him that they were amazed "how many stores were secured and people were locked in place" upon entering the mall to look for the shooter.